1. Field of the Invention
This invention relates generally to a device and method for enhancing the accuracy of a current measuring apparatus, and more particularly where the current measuring apparatus uses optical sensing technology.
2. Description of the Related Art
Conventional high current (up to about 500 KA) measurement equipment uses Hall effect technology to measure the magnetic field around a conductor. Electricity carried through a conductor produces a magnetic field that varies with current, and conventional measurement equipment uses an electronic sensor (Hall sensor) that varies its output voltage in response to changes in magnetic field density. A Hall sensor in close proximity to the conductor can thus be used to effectively measure the current without interrupting the circuit or making electrical contact with the conductor. Typically, the Hall sensor is integrated with a wound core that surrounds the conductor to be measured. Conventional Hall technology current measuring equipment has the disadvantages of being expensive, large, heavy and time-consuming to install.
Optical devices can also be used to measure current. Such devices are interferometers that utilize the Faraday effect, in which there is an interaction between light and the magnetic field produced around the conductor, to measure current. Existing technology for carrying out this measurement does not have sufficient accuracy under all circumstances. The reason for this is explained below.
An optical interferometer of the type described, such as the Fiber Optic Current Sensor made and sold by Nxtphase, works on the principle that the speeds of right handed circularly polarized (RHCP) and left handed circularly polarized (LHCP) light waves are oppositely affected by a magnetic field. A fiber optic circuit is arranged in such a way that two beams, one that is RHCP and one that is LHCP, are sent through an optical fiber that extends through the magnetic field around the conductor, and the total phase difference accumulated between the two beams is measured. The total phase difference is proportional to the line integral of the magnetic field along the path of the sensing fiber. Thus, by extending the optical fiber around the current carrying conductor an integral number of times, the sensor measures the closed path integral of the magnetic field around the conductor. By Ampere's Law, this is equal to the current carried in the conductor.
The principle of reciprocity ensures that common mode effects are cancelled, and thus path non-idealities cannot create a phase difference between the two beams. However, the influence of the magnetic field through the Faraday effect is non-reciprocal. This is because the sensing path is terminated in a mirror, at which a RHCP beam is converted to a LHCP beam upon reflection, and vice versa. Thus, the outbound RHCP light beam returns from the mirror as a LHCP beam, and vice versa. Both beams travel through the sensing path as both RHCP and LHCP beams, only in opposite directions. As the sense of circular polarization is reversed upon reflection at the mirror, so also is the direction of propagation relative to the magnetic field. Thus, when compared to a unidirectional sensor configuration, the “round trip” configuration provides that reciprocal non-idealities are cancelled and the magnetic field effect is doubled.
The phase shift caused by the magnetic field is a function of the current flowing in the conductor, and there are two main properties that affect how much phase shift is measured for a given current, also called the “scaling” of the current sensor. The first of these is the quality of the quarter wave plate. The quarter wave plate defines the beginning of the sensing region, where the beams are polarized, and the end of the sensing region for the returning light. An imperfect quarter wave plate gives rise to impure beams, and consequently a change in the scaling of the sensor.
The second property that affects the scaling of the sensor is the magnetic sensitivity of the sensing fiber itself. The sensitivity of the fiber to the effects of a magnetic field is described by the Verdet constant of the fiber.
The properties described above that affect the scaling of the sensor (the quarter wave plate quality and the Verdet constant of the fiber) are functions of temperature. Current sensors are used for the measurement of large DC currents, and the conductors that carry these currents are large and generate significant heat. Because of this, there can be large temperature gradients around the conductor, which reduces the ability of the current sensor to maintain an accurate scale factor.
It is currently known to measure the temperature of components of an optical current sensor, and correct any error in the current sensor output caused by an increase in temperature. However, there are still problems with accuracy in determining the measured current.